In typical surface-mount circuit board manufacturing operations, a stencil printer is used to print solder paste onto a circuit board. Typically a circuit board having a pattern of pads or some other usually conductive surface onto which solder paste will be deposited is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. In some prior art systems, an optical alignment system is used to align the circuit board with the stencil. Examples of optical alignment systems for stencil printers are described in U.S. Pat. No. 5,060,063, issued Oct. 21, 1991 to Freeman, and in U.S. Pat. No. Re. 34,615 issued Jan. 31, 1992, also to Freeman, each of which is incorporated herein by reference.
The solder paste print operation can be a primary source of defects in surface mount assembly. Thus, processes have been developed to inspect a substrate after solder paste has been deposited thereon to determine whether the solder paste has been properly applied to conductive pads located on the substrate, usually, but not limited to, conduction pads on a printed circuit board. For example, in some prior art systems, the optical alignment system mentioned above also includes a vision inspection system to inspect the substrate.
Automated machine-based techniques for assessing print defects, however, are only able to isolate and quantify tangible characteristics. Reliable methods are still needed to classify and weigh the significance of print defects as they relate to the process, to provide meaningful output and to define realistic and useful process limits. Although subsequent processes certainly play a part in determining the quality of the final printed circuit board (PCB assembly), detection and accurate assessment of bridge and other defects, as printed, can provide the most direct feedback for process control of appropriate print functions.